1. Field of the Invention
The present invention relates to a variable magnification optical system for use in an optical unit or the like, and to an image taking apparatus provided with this variable magnification optical system.
2. Description of the Prior Art
In recent years, following the widespread use of personal computers (PCs), digital cameras (image taking apparatuses) capable of easily taking in an image have been widespread. Thus, as is the case with cameras using a silver halide film (silver halide film cameras), there has been a demand imposed on such digital cameras for downsizing (slimming-down) and performance enhancement (for example, high magnification variation function and high aberration correction function).
For example, image taking apparatuses disclosed in patent publications 1 and 2 employ an optical prism capable of bending the optical path to satisfy the demand for downsizing. However, these image taking apparatuses assume a zoom ratio (magnification variation ratio) of approximately 3×. Therefore, to satisfy the demand for performance enhancement, it is preferable that the image taking apparatus provide a higher magnification ratio.
On the other hand, an image taking apparatus disclosed in patent publication 3 has lens units with optical power arrangement of positive, negative, positive, and positive, and includes, in the lens unit located at the first position from the object side toward the image side, an optical prism having a positive optical power. This image taking apparatus, as is the case with the image taking apparatuses in patent publications 1 and 2, achieves downsizing through the use of the optical prism. Moreover, this image taking apparatus assumes a zoom ratio of approximately 5×, and thus is a compact, high-performance image taking apparatus.
[Patent publication 1] JP-A-2003-202500                Disclosure date: Jul. 18, 2003        
[Patent publication 2] JP-A-2003-302576                Disclosure date: Oct. 24, 2003        
[Patent publication 3] JP-A-2004-170707                Disclosure date: Jun. 17, 2004        
However, for the image taking apparatus of this patent publication 3, the optical prism is formed of a relatively high-dispersion material. Thus, in a case where the optical prism is decentered when built into an optical system (variable magnification optical system) inside the image taking apparatus (for example, when a surface of the optical prism is inclined from a desired position), there is a risk that a relatively large scale of chromatic aberration, attributable to the decentering, occurs so as to achieve a high zoom ratio.